1. Field of the Invention
The present invention relates to a device which provides a conduit for patient respiratory gases and an optical channel for the passage of infrared radiation from a respiratory gas analyzer through the gases, and more particularly, to a disposable mainstream anti-fog airway adapter which has infrared transmissive windows treated by an anti-fog agent to prevent fogging by the patient's breath. The present invention also includes several techniques for forming the anti-fog windows and sealing them to the adapter body so that the adapter can be used with a mainstream respiratory gas analyzer.
2. Brief Description of the Prior Art
Mainstream respiratory gas analyzers provide a measurement of a patient's inhaled and exhaled gases by directing a beam of infrared energy across the patient's respiratory circuit and measuring the infrared absorption at the respective infrared frequencies. Mainstream respiratory gas analyzers are preferably located very close to the patient's mouth to provide accurate gas concentration measurements. Such measurements are very valuable at the time of intubation to determine if the endotracheal tube is properly located and during long term mechanical ventilation to determine the status of the patient's cardiopulmonary system. The patient's breathing circuit is a closed circuit, and the infrared energy enters and exits the respiratory gas stream by passing through "windows" which are typically located in the airway adapter. The "windows" are sections of a material that will pass the required wavelengths of infrared energy while maintaining an airtight seal of the patient's respiratory circuit.
Such infrared respiratory gas analyzers function by passing light of a specific wavelength (typically infrared) through a gas and measuring the amount of light that is absorbed. Such a respiratory gas analyzer is disclosed by Solomon in an article entitled "A Reliable, Accurate CO.sub.2 Analyzer for Medical Use," Hewlett-Packard Journal, September 1981, pp. 3-21, for example. Solomon therein describes the HP CO.sub.2 analyzer model 47210A Capnometer, which measures the amount of carbon dioxide in a patient's breath for medical diagnostic purposes. The HP model 47210A Capnometer is comprised of an airway adapter, a sensor and a processor box. The airway adapter described by Solomon is a hollow aluminum casting with sapphire windows which is inserted in series with the ventilator plumbing and is used to keep the patient's respiratory gases from coming into contact with the sensor mechanism. The sensor is snapped over the airway adapter windows, and the measurement is made directly on the airway through which the patient is breathing. The sensor contains all the optical components necessary to make the infrared measurement and is connected to the processor box by a cable. The processor box powers the sensor, processes the return signal, and presents the data via an LED display.
The airway adapter of the HP model 47210A Capnometer is made of aluminum so that it can be sterilized and adapted to meet a number of other critical requirements such as small size, light weight, stable infrared path length, ruggedness and uniformity. It has become desirable to develop an airway adapter which is disposable and hence useful for preventing infection through cross-contamination. However, such a disposable airway adapter must still meet numerous critical requirements, such as those just enumerated, in order to provide sufficient accuracy of measurement.
A respiratory gas analyzer utilizing an interchangeable, low cost disposable airway adapter suitable for single patient use is described in U.S. Pat. Nos. 5,081,998 and 5,095,913, while an embodiment of a disposable airway adapter for such a system is described in U.S. Pat. No. 5,067,492, all of which are assigned to the same assignee as the present invention. As described in those patents, the airway adapter must provide a sealed, tubular passage for the unobstructed flow of respiratory gases while also providing an optical path through the respiratory gases which are in the portion of the airway adapter between the windows. The windows must be transparent to the infrared wavelengths used to analyze the constituent gases, and the optical path length defined by the airway adapter and infrared absorption fingerprint of the infrared windows must remain constant among interchangeable adapters for accuracy of measurement. Also, in order for the airway adapter to be disposable, means must be provided for accurately locating the airway adapter in the respiratory gas analyzer such that the optical path length through the respiratory airstream remains a constant length. For example, as described in U.S. Pat. No. 5,067,492, the airway adapter may snap fit into the respiratory gas analyzer.
The airway adapter described in U.S. Pat. No. 5,067,492 is comprised of a tubular body and thin plastic windows. The adapter body is preferably injection molded out of polycarbonate or styrene acrylonitrate (SAN). The plastic windows, on the other hand, are preferably fabricated from a polyester film (such as DuPont. Mylar.RTM. 100 XM963) with a thickness of 0.001 inch +/-0.0005 inch. The polyester film windows are then sealed to the adapter body so that there are no gas leaks and so that the film is maintained in tension. This tensile "prestressing" prevents wrinkles from developing under reasonable temperature excursions and prevents the windows from bulging during reasonable pressure excursions within the respiratory gas circuit, for the calibration of the gas analyzer system could be adversely affected if the windows were allowed to bulge more than the allowable tolerance of the optical path length.
However, a problem has been encountered with "mainstream" respiratory gas detectors of the type described in the aforementioned patents. Namely, for enhanced patient care for long-term ventilated patients, the inspired gases are often heated to body temperature and humidified. In addition, even in short cases where humidification is not required, the patient's exhaled gas is almost fully saturated with water vapor at body temperature. This water vapor has been found to condense on the airway adapter and on the infrared windows, thereby posing an obstacle to the transmission of the infrared energy through the windows. The condensed water is troublesome to measurements because it absorbs infrared energy, thereby reducing the signal strength. In addition, the water often condenses in droplets which act as small lenses which distort the infrared energy and further increase the attenuation of the infrared signal due to the physical thickness of the droplets. The present inventors have set out to solve this problem.
Traditional solutions to the condensation problem involve heating the windows to a temperature above body temperature to prevent condensation. This solution is effective but has several inherent problems. For example, the heat takes time to develop after the instrument is turned on. This makes "instant on" operation of the analyzer impossible. Since one of the uses of the analyzer is verification of proper intubation, the ability to simply turn the instrument on and use it without "warm-up" is valuable, and having to wait for "warm-up" may render the instrument useless in a critical intubation procedure. In addition, generation of the heat requires power. Since some monitors are used in a "transport" mode where the monitoring equipment is powered by internal batteries, the use of additional power to heat the windows is a disadvantage. This requires the batteries to be larger and heavier and/or the useful life of the instrument operating on the battery to be reduced. Moreover, the heat can be dangerous to the patient. Heating the windows to above body temperature could cause patient burns if the heated analyzer came into contact with the patient's skin. Furthermore, the warm, dry nature of the infrared window maintained at an elevated temperature promotes the adhesion of other patient secretions such as blood and mucus if they are coughed up into the airway. These contaminants can block the :infrared energy and render the analyzer inoperative.
Accordingly, the present invention is designed to solve the aforementioned problems caused by water condensation and the like on the windows of a disposable airway adapter. The present inventors know of no other suitable prior art disposable airway adapter and hence believe that no one has previously recognized the problem which is to be solved by the present invention. Thus, the present inventors also know of no prior art technique besides that described above for preventing the condensation of water droplets from a patient on the windows of a mainstream airway adapter. The present invention is thus believed to be the first attempt to meet such needs.